The present invention relates to a base station mutual synchronization and monitoring system which is used in a communication network system including a plurality of base stations and a large number of mobile stations, e.g., portable telephones, to effect mutual synchronization and monitoring between the base stations.
TDMA (Time Division Multiple Access) is a technique whereby a plurality of stations are assigned a plurality of channels, respectively, which are obtained by dividing the time base into a plurality of periodically repeated blocks, to effect communication. In the TDMA system, it is important to synchronize channels on the time base, which are assigned to respective base stations. If the channels are not satisfactorily synchronized, the timing at which a signal is transmitted from one base station overlaps the signal transmission timing in another base station, resulting in a lowering of the channel utilization efficiency.
Conventional techniques proposed to overcome the above disadvantage include a method wherein a synchronizing signal is sent from a central station to each base station in a hardware manner. However, this method requires large transmission power and also involves the problem of the difference in radio wave propagation time between a remote base station and a base station close to the central station. In addition, it is necessary to change the control for base stations using different systems.
Techniques proposed to solve these problems include a method disclosed in Japanese Patent Application Laid-Open (KOKAI) No. 04-287532 (1992). According to the disclosed method, no central station is installed, but base stations mutually receive a signal corresponding to each other's synchronizing signal, and each base station compares the received signal with its own transmitted signal, thereby correcting any error in synchronization of the transmitted signal. Thus, synchronization between the base stations is established in a self-distributed manner.
However, the above-described system requires each base station to have a plurality of receivers in order to receive radio waves from other base stations at different timings while maintaining the timing for transmitting its own radio wave.
In addition, since each individual base station shifts its own transmitting timing so as to bring the synchronization error into convergence, an error may be produced in synchronization with a mobile station (personal station) which has been synchronized with the base station concerned.
Further, correction of the transmitting timing which is made to achieve convergence of synchronization newly produces a synchronization error, which causes undulatory vibration. Accordingly, it takes a long time to bring the synchronization into convergence.
Furthermore, in the above-described communication system, each individual base station is not constantly monitored. Therefore, occurrence of an abnormality is discovered by a report from a user or a maintenance manager or by detecting a lowering in the communication system utilization efficiency due to the abnormality.
As a testing method, for example, loop-back testing in which a signal is passed through the transmitting unit and returned to the receiving unit has heretofore been carried out for each individual base station. With this testing method, however, it is difficult to judge whether or not normal radio transmission is actually performed in the air and whether or not normal radio reception can be effected. Therefore, the conventional practice is to connect a special measuring device to each base station for detection and analysis of an abnormal phenomenon to thereby perform monitoring and testing.
However, since the conventional system has no function of monitoring at all times, occurrence of an abnormality cannot be discovered until a user reports it as a problem in the ordinary service.
In the case of a problem which is difficult for a user to judge, e.g., a lowering in the transmission output, the problem cannot promptly be discovered and dealt with.
In addition, it is necessary in order to effect abnormality monitoring to provide a special device for monitoring, together with a radio signal and communication procedure therefor, which invites an increase in the cost for the maintenance operation.